1. Field of the Invention
This invention relates to heater abnormality detecting circuits, and more particularly to a CRT heater abnormality detecting circuit used, for example, for television receivers, display units and so on.
2. Description of the Prior Art
In a conventional heater abnormality detecting circuit 1 shown in FIG. 3, normality or abnormality is determined based on a level of a bias voltage detected by a micro-computer 2. That is, if the heater 3 is normal, a fly back pulse (FB pulse) extracted on a heater winding 4 is supplied to the heater 3 through a heater register R1, and also rectified by a diode D1 so that a voltage taken by an electrolytic capacitor C1 is voltage-divided by resistors R2 and R3. Accordingly, an electric current flows through the resistor R3 and a diode D2 is turned on, thereby causing a bias current flowing through the resistor R3. That is, there is a decrease in level of a bias voltage detected by the micro-computer 2.
On the other hand, in the event that the heater be short circuited, an FB pulse extracted on the heater winding 4 is supplied to the heater resistor R1, and also to a ground through the short-circuited heater 3. Due to this, the diode D2 will not turn on with a result that the micro-computer 2 detects a bias voltage that is dropped in voltage from a bias B through a resistor R4. Accordingly, the bias voltage is higher in level as compared with the case that the heater 3 is normal.
In this prior art, however, the FB pulse is rectified by the diode D1 so that a voltage taken by electrolytic capacitor C1 is voltage-divided by the resistors R2 and R3. Also, the diode D2 is provided in order to prevent a bias current from flowing into the micro-computer 2 (prevention against reverse current flow). Thus there is a necessity to provide an element to extract as a voltage an FB pulse, an element to prevent against current reverse flow, and so on Thus, the number of parts required is increased, resulting in mounting up of cost.
Furthermore, the electrolytic capacitor C1 and the resistor R2 might form a CR circuit. Due to a presence of a time constant of this CR circuit, the determination of normality or abnormality on the heater 3 is delayed by several msecs. Thus there is a disadvantage that there is delay in detecting an abnormality in the heater 3 resulting in possible smoking from the heater resistor R1.